This invention relates to high frequency operation of an electric discharge lamp and, more particularly, to an improved high frequency ballast circuit for starting and operating a so-called energy saver discharge lamp or the like.
Circuits for starting and ballasting a gas discharge lamp are generally required to provide stable and efficient operation thereof. During normal operation, the discharge lamp exhibits a negative impedance characteristic. A ballast circuit is therefore required in order to provide a positive series impedance or other current limiting mechanism to balance the negative impedance characteristic of the lamp and thereby provide stable operation. The voltage required to initiate a discharge in such a lamp is generally substantially higher than the normal operating voltage of the lamp. An auxiliary starting circuit may be used to provide the high starting voltage to initiate the lamp discharge. The lamp ballasting function has usually been provided by an inductor or resistor connected in series with the discharge lamp.
It is known that high frequency operation of electric discharge lamps provides several unique advantages over low frequency, e.g. 60 Hz, operation thereof. For example, high frequency operation of a discharge lamp provides higher efficacy than low frequency operation while simultaneously permitting the use of reactive components of much smaller size and therefore reduced cost. High frequency operation often results in an improvement in the circuit power factor and a significant reduction of power losses in the ballast.
The typical "energy saver" type of electric discharge lamp normally contains a conductive film or strip on the internal surface of the lamp which allows the lamp to start and operate with a standard 60 Hz supply voltage even though the lamp may have a Krypton-neon or Krypton-argon fill gas. A serious problem with all energy saver type lamps which incorporate this internal conductive film or strip is that they are extremely difficult, if not impossible, to start when used in conjunction with a high frequency ballast. It is believed that at the operating frequencies (approximately 15 KHz-50 KHz) of standard high frequency ballasts, the AC voltage applied across the lamp electrodes is capacitively coupled between the electrodes and the internal conductive coating on the lamp so as to effectively apply a short circuit across the lamp electrodes and thereby prevent ionization of the fill gas within the lamp envelope beyond the vicinity of the electrodes. This occurs because, as the supply frequency is increased, the impedance between each electrode and the conductive wall decreases to a value such that the electrode-to-wall potential drop is insufficient to permit full ionization of the fill gas within the lamp. As a result, the lamp will not ignite. However, in order to obtain maximum efficacy and energy savings with energy saver lamps, it is desirable to operate them by means of high frequency - high efficiency drive circuits provided that a feasible method to start them can be found.
A static inverter for operating a gas discharge lamp, in which the inverter will oscillate at a first frequency during the lamp pre-ignition period (e.g. 22 KHz) and then will automatically increase its oscillating frequency to approximately 27 KHz during normal operation of the lamp, is described in U.S. Pat. No. 4,245,177 issued Jan. 13, 1981 in the name of N. A. Schmitz. However, the inverter disclosed therein is not concerned with the special problems involved in the high frequency ignition of energy saver type discharge lamps. Nor is there any indication that the cause of the aforesaid ignition problem was even recognized, or its solution even a remote consideration in the design of the Schmitz static inverter.
In U.S. Pat. No. 4,060,751 issued Nov. 29, 1977 to T. E. Anderson, there is described a dual mode solid state inverter circuit for starting and ballasting a gas discharge lamp. Before ignition of the discharge lamp, an AC inverter operates at the resonant frequency of a series resonant LC circuit so that a ringing voltage developed across the capacitor builds up to a level sufficient to ignite the lamp. Subsequently, the inverter frequency is controlled as a function of the load current sensed by a current detector so as to limit the lamp current and thereby provide the normal ballast function required by a discharge lamp. Other variable frequency inverter circuits for regulating the current in a discharge lamp are described in U.S. Pat. No. 4,220,896 issued Sept. 2, 1980 to D. A. Paice and in U.K. Patent 1,578,037 published Oct. 29, 1980 in the name of L. H. Walker.